Self-aligned spacer for cut-last transistor fabrication

ABSTRACT

Semiconductor devices and methods of forming the same include laterally etching a dummy gate to recess the dummy gate underneath a spacer layer, such that the spacer layer overhangs the dummy gate. A sidewall of the dummy gate is nitridized. The dummy gate is etched away without removing the nitridized sidewall. A gate is formed within a boundary defined by the nitridized sidewall. A conductive contact to the gate is formed.

BACKGROUND

Technical Field

The present invention generally relates to semiconductor devicefabrication and, more particularly, to selective nitridation of a dummygate to prevent shorting between contacts.

Description of the Related Art

When gate cut last processes are used and silicon nitride is depositedto fill the gap, problems can arise in the middle-of-line (MOL). Inparticular, structures are formed in the gate cut, which can be quitenarrow. Existing processes form, for example, contacts and power railsin a silicon nitride filler that is in contact with the gate. Whencontacts are formed through this silicon nitride filler, it is possibleto over-etch the material and breach the lining such that, for example,the power rail can short-circuit to the gate. This decreases deviceyield and device reliability.

Other structures use a silicon nitride liner around only the gate.However, this structure is generally formed using an anisotropic etch toremove excess liner material from horizontal surfaces of the device.Such an etch will remove some material from the vertical portions of theliner as well, resulting in a tapered profile and a thinner liner at thetop of the gate. This thinned liner is susceptible to short circuits to,e.g., a nearby power rail.

SUMMARY

A method of forming a semiconductor device includes laterally etching adummy gate to recess the dummy gate underneath a spacer layer, such thatthe spacer layer overhangs the dummy gate. A sidewall of the dummy gateis nitridized. The dummy gate is etched away without removing thenitridized sidewall. A gate is formed within a boundary defined by thenitridized sidewall. A conductive contact to the gate is formed.

A method of forming a semiconductor device includes etching away asidewall spacer formed on a polysilicon dummy gate. The dummy gate islaterally etched to recess the dummy gate underneath an upper spacerlayer, such that the upper spacer layer overhangs the dummy gate. Asidewall of the dummy gate is nitridized to form a silicon nitridesidewall having a uniform thickness. The dummy gate is etched awaywithout removing the nitridized sidewall. A dummy gate dielectric isetched away, leaving a dummy gate dielectric remnant directly underneaththe nitridized sidewall. A gate dielectric is formed over one or moresemiconductor fins and over the nitridized sidewall. A gate is formedwithin a boundary defined by the nitridized sidewall. A passivatingdielectric layer is formed over the gate. A conductive contact to thegate is formed. A power rail is formed in contact with the nitridizedsidewall.

A semiconductor device includes one or more semiconductor fins. A gateis formed over the one or more semiconductor fins. A vertical sidewallis formed at a perimeter of the gate, the vertical sidewall having auniform thickness along its height. A power rail is formed in contactwith the vertical sidewall.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 2 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 3 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 4 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 5 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 6 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 7 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 8 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 9 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 10 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 11 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 12 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 13 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments;

FIG. 14 is a cross-sectional view of a step in the formation of asemiconductor device in accordance with the present embodiments; and

FIG. 15 is a block/flow diagram of a method of forming a semiconductordevice in accordance with the present embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention make use of self-aligned nitridespacers on a dummy gate material to establish a barrier againstshort-circuiting the final gate structure to other contacts. Inparticular, the nitride spacers of the present invention are not exposedto an anisotropic etch that might cause thinning of the spacers at thetop. Instead, the spacers have a roughly uniform thickness along theirentire length.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a cross-sectional view of astep in forming a semiconductor device is shown. This figure shows anin-progress device, built on a semiconductor substrate 102. Thesemiconductor substrate 102 may be a base substrate, with no furtherchip layers beneath it, or may alternatively be an intermediatesubstrate having one or more further circuit layers underneath. Thesemiconductor substrate 102 may be a bulk-semiconductor substrate. Inone example, the bulk-semiconductor substrate may be asilicon-containing material. Illustrative examples of silicon-containingmaterials suitable for the bulk-semiconductor substrate include, but arenot limited to, silicon, silicon germanium, silicon germanium carbide,silicon carbide, polysilicon, epitaxial silicon, amorphous silicon, andmulti-layers thereof. Although silicon is the predominantly usedsemiconductor material in wafer fabrication, alternative semiconductormaterials can be employed, such as, but not limited to, germanium,gallium arsenide, gallium nitride, cadmium telluride and zinc selenide.Although not depicted herein, the semiconductor substrate 102 may alsobe a semiconductor on insulator (SOI) substrate.

The semiconductor substrate 102 has one or more semiconductor fins 104formed thereon. The semiconductor fin 104 may itself be formed by anyappropriate lithographic process including, e.g., a photolithographicmask and etch. A pattern is produced by applying a photoresist to thesurface of the semiconductor substrate 102. The photoresist is exposedto a pattern of radiation that causes a chemical reaction within thephotoresist. The pattern is then developed into the photoresistutilizing a resist developer. Once the patterning of the photoresist iscompleted, the sections covered by the photoresist are protected whilethe exposed regions are removed using a selective etching process thatremoves the unprotected regions. The photoresist may also be removedafter patterning is complete. In one embodiment, a hardmask may be usedto form the semiconductor fin 104. The mask may be formed by firstdepositing a dielectric hardmask material, like silicon nitride orsilicon dioxide atop a layer of semiconductor layer and then applying aphotoresist pattern to the hardmask material using a lithographyprocess. The photoresist pattern is then transferred into the hardmaskmaterial using a dry etch process. Next the photoresist pattern isremoved and the pattern is transferred into the semiconductor materialduring a selective etching process, such as reactive ion etching (RIE).The remaining mask material may be removed by a wet or dry etch.

RIE is a form of plasma etching in which during etching the surface tobe etched is placed on a radio-frequency powered electrode. Moreover,during RIE the surface to be etched takes on a potential thataccelerates the etching species extracted from plasma toward thesurface, in which the chemical etching reaction is taking place in thedirection normal to the surface. Other examples of anisotropic etchingthat can be used at this point of the present invention include ion beametching, plasma etching or laser ablation. Alternatively, thesemiconductor fin 104 can be formed by other patterning techniques suchas spacer image transfer.

A dielectric barrier layer 106 is formed on the semiconductor substrate102 and may be formed from any appropriate insulating or dielectricmaterial. A dummy dielectric 108 is formed conformally over thedielectric barrier layer 106 and a dummy gate 110 is formed on the dummydielectric 108. The dummy dielectric 108 may be formed from anyappropriate dielectric material including, e.g., silicon dioxide. Thedummy oxide 108 should be formed from a different material from thedielectric barrier layer 106 and should have etch selectivity withrespect to the dielectric barrier layer 106. It is specificallycontemplated that the dummy gate 110 may be formed from polysilicon, butany appropriate material may be used in its place.

A spacer layer 112 is formed over the dummy gate 110. It is specificallycontemplated that the spacer layer 112 may be formed from siliconnitride, although any other appropriate dielectric or hardmask materialmay be used instead. A dielectric layer 114 is formed over the spacerlayer 112 from, e.g., silicon dioxide, and a liner dielectric layer 116is formed over the dielectric layer 114 from, e.g., silicon nitride. Itis specifically contemplated that the liner dielectric layer 116 may beformed from the same material as the spacer layer 112. An inter-layerdielectric 118 is formed around the liner dielectric layer 116.

The device has been polished down with, e.g., a chemical mechanicalplanarization (CMP) process that stops on the liner dielectric layer116. CMP is performed using, e.g., a chemical or granular slurry andmechanical force to gradually remove upper layers of the device. Theslurry may be formulated to be unable to dissolve, for example, theliner dielectric material, resulting in the CMP process's inability toproceed any farther than that layer.

Referring now to FIG. 2, a cross-sectional view of a step in forming asemiconductor device is shown. A first mask layer 202 and a second masklayer 204 are deposited over the liner dielectric layer 116 and theinter-layer dielectric 118. It is specifically contemplated that thefirst mask layer 202 may be formed from the same material as the linerdielectric 118 (e.g., silicon nitride) and that the second mask layer204 may be formed from the same material as the inter-layer dielectric118 (e.g., silicon dioxide).

The first and second mask layers 202 and 204 may be formed by anyappropriate deposition process including, e.g., chemical vapordeposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), or gas cluster ion beam (GCIB) deposition. CVD is adeposition process in which a deposited species is formed as a result ofchemical reaction between gaseous reactants at greater than roomtemperature (e.g., from about 25° C. about 900° C.). The solid productof the reaction is deposited on the surface on which a film, coating, orlayer of the solid product is to be formed. Variations of CVD processesinclude, but are not limited to, Atmospheric Pressure CVD (APCVD), LowPressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), and Metal-Organic CVD(MOCVD) and combinations thereof may also be employed. In alternativeembodiments that use PVD, a sputtering apparatus may includedirect-current diode systems, radio frequency sputtering, magnetronsputtering, or ionized metal plasma sputtering. In alternativeembodiments that use ALD, chemical precursors react with the surface ofa material one at a time to deposit a thin film on the surface. Inalternative embodiments that use GCIB deposition, a high-pressure gas isallowed to expand in a vacuum, subsequently condensing into clusters.The clusters can be ionized and directed onto a surface, providing ahighly anisotropic deposition.

Referring now to FIG. 3, a cross-sectional view of a step in forming asemiconductor device is shown. The first and second mask layers 202 and204 are opened to form gap 302. In particular, two separate etches maybe used, with the first etch removing material from the second masklayer 204 and the first etch removing material from the first mask layer202. The gap 302 has a stepped profile, with the opening in the firstmask layer 202 being smaller than the opening in the second mask layer204. It is specifically contemplated that the gap 302 has a width thatextends past the vertical sides of the liner dielectric layer 116. Thegap 302 exposes the spacer layer 112.

The etches may be performed photolithographically. In one exemplaryetch, a photoresist mask is formed overlying the second mask layer 204material. The exposed portions of the second mask layer 204, which arenot protected by the photoresist mask, are removed using a selectiveetch process. To provide the photoresist mask, a photoresist layer isfirst positioned on the second dielectric layer 204. The photoresistlayer may be provided by a blanket layer of photoresist material that isformed utilizing a deposition process such as, for example, CVD, PECVD,evaporation, or spin-on coating. The blanket layer of photoresistmaterial is then patterned to provide the photoresist mask utilizing alithographic process that may include exposing the photoresist materialto a pattern of radiation and developing the exposed photoresistmaterial utilizing a resist developer. A similar etch may be used toopen the first mask layer 202.

Referring now to FIG. 4, a cross-sectional view of a step in forming asemiconductor device is shown. An anisotropic etch, such as RIE, is usedto remove material from dielectric layer 114 and inter-layer dielectric118. The etch exposes more of the spacer layer 112 and exposes the sideof the liner dielectric 116.

Referring now to FIG. 5, a cross-sectional view of a step in forming asemiconductor device is shown. An anisotropic etch, such as RIE, is usedto remove material from the spacer layer 112 and the liner dielectric116. The etch exposes a lateral side of the dummy gate 110.

Referring now to FIG. 6, a cross-sectional view of a step in forming asemiconductor device is shown. An isotropic etch is used to removematerial from the dummy gate 110. The isotropic etch may include a wetor dry chemical etch that removes the exposed material from the dummygate 110 and furthermore etches the dummy gate 110 underneath theremaining spacer layer 112, creating an overhang 602.

Referring now to FIG. 7, a cross-sectional view of a step in forming asemiconductor device is shown. A selective nitridation process is usedto nitridize the lateral surface of the dummy gate 110 and form sidewall702. In the case where the dummy gate 110 is formed from polysilicon,the nitridation process forms a layer of silicon nitride from thelateral surface of the dummy gate 110. It should be noted that thesidewall 702 forms around the entire dummy gate 110, including thesurfaces that run parallel to the page (not shown). The sidewall 702 hasa roughly uniform thickness along its entire height, and this thicknessis maintained throughout the fabrication.

It is specifically contemplated that thermal nitridation may be used toform the sidewall 702. Thermal nitridation may include exposing theexposed surface of the dummy gate 110 to a nitrogen-containing gas.Examples of nitrogen-containing gases include, but are not limited tonitrogen gas, ammonia, ammonium, nitric oxide, and mixtures thereof. Thenitrogen-containing gas can be pure or can be diluted with hydrogen gasor an inert gas such as helium, neon, argon, and mixtures thereof. Thenitrogen concentration in the nitridizing gas may be from about 10% toabout 100%, with a range of about 50% to about 80% being preferred. Inone embodiment, thermal nitridation is performed at a temperaturebetween about 50° C. and about 450° C., with a range between about 100°C. and about 300° C. being preferred. The layer of nitridation 302varies depending on the type of nitrogen-containing gas and thetemperature at which the thermal nitridation is performed. The depth ofthe layer of nitridation 702 may be between 50 nm and about 5 nm.

Referring now to FIG. 8, a cross-sectional view of a step in forming asemiconductor device is shown. A flowable dielectric material isdeposited (e.g., the same material as is used in the inter-layerdielectric 118 and in the dielectric layer 114). It is specificallycontemplated that the dielectric material may be, e.g., silicon dioxideand that the flowable oxide may be deposited in liquid form andsubsequently solidified. Alternative deposition processes, such as ahigh-density plasma oxide deposition or CVD, may be used instead. Afterdeposition, a CMP process is used to polish the deposited dielectricmaterial down to the level of the spacer layer 112, forming passivatingdielectric 802.

Referring now to FIG. 9, a cross-sectional view of a step in forming asemiconductor device is shown. The dummy gate 110 and the spacer layer112 are etched away, exposing the dummy dielectric 108 and leaving thesidewall 702 in place. The spacer layer 112 is etched away using, e.g.,a directional dry etch that stops when the dummy gate 110 has beenexposed. This removes the spacer material from horizontal surfaceswithout substantially affecting the material in the vertical sidewall702.

Referring now to FIG. 10, a cross-sectional view of a step in forming asemiconductor device is shown. the dummy dielectric 108 is removed usingany appropriate etch and a gate dielectric layer 1002 is conformallydeposited over the fins 104 using, e.g., CVD or any other appropriatedeposition process. It is specifically contemplated that the gatedielectric layer 1002 may be a high-k dielectric material, which isdefined to be any material having a dielectric constant higher than thatof silicon dioxide. Exemplary high-k dielectric include, e.g., hafniumdioxide, zirconium dioxide, aluminum oxide, titanium dioxide, lanthanumoxide, strontium titanium oxide, lanthanum aluminum oxide, yttriumoxide, and mixtures thereof.

Referring now to FIG. 11, a cross-sectional view of a step in forming asemiconductor device is shown. A gate material is deposited over thegate dielectric layer 1002. The gate material may be any appropriateconductor including, e.g., tungsten, nickel, titanium, molybdenum,tantalum, copper, platinum, silver, gold, ruthenium, iridium, rhodium,rhenium, titanium nitride, titanium carbide, aluminum doped titaniumcarbide, alloys of any of the preceding metals, doped polysilicon andpolysilicon-germanium alloy materials, and polycide materials (e.g.,doped polysilicon/metal silicide stack materials). After depositing thegate material, CMP is used to polish down to the passivating layer 802to form gate 1102.

Referring now to FIG. 12, a cross-sectional view of a step in forming asemiconductor device is shown. The gate 1102 and the gate dielectric1002 are recessed by, e.g., an RIE process to form recessed gate 1202. Alayer of dielectric material (e.g., a nitride similar to the nitride ofsidewall 702) is conformally deposited. The deposited dielectricmaterial may then be polished down to the level of the passivating layer802 to form upper spacer 1204 which may be formed continuous with thesidewall 702. The upper spacer 1204 will prevent source/drain contacttrenches from shorting to the gate in the finished device.

At this stage, contacts may be formed in source/gate regions of the fins104, which are not shown in the present figures and which are onportions of the fins 104 that are not covered by the gate 1102. Suchcontacts may be formed by depositing a metal on the fins 104 and forminga silicide through annealing the metal, such that the metal diffusesinto the body of the fins 104.

Referring now to FIG. 13, a cross-sectional view of a step in forming asemiconductor device is shown. Additional dielectric material (e.g., anoxide similar to the oxide of the passivating layer 802, such as silicondioxide) is deposited. The additional dielectric material covers thedevice and forms passivating layer 1302. It should be noted that thispassivating layer 1302 is formed from a material that has etchselectivity with respect to the upper spacer 1204 and sidewall 702.

Referring now to FIG. 14, a cross-sectional view of a step in forming asemiconductor device is shown. Holes are etched into the passivatinglayer 1302 and conductive material (e.g., a contact metal) is depositedin the holes to form conductive contacts. A first contact 1402 is shownas contacting the gate 1202, while a second conductive contact 1404 maybe used as a power rail. The holes may be formed by any appropriateanisotropic etch such as, e.g., RIE.

It should be noted that the different materials of the sidewalls 702 (aswell as the upper spacer 1204) and the passivating layer 1302 makes itpossible to cleanly etch the holes in the passivating layer 1302 withoutrisk of over-etching, thereby preventing shorts between the contacts1402/1404 and the gate 1202. In one specific embodiment, the sidewall702 and upper spacer 1204 are formed from silicon nitride and thepassivating layer is formed from silicon dioxide. Notably, the secondconductive contact 1404 may be formed quite close to the gate 1202 but,due to the uniform thickness of sidewall 702, the likelihood of ashort-circuit is significantly decreased.

As noted above, the sidewall 702 runs along the perimeter of the gate1202, including on sides that run parallel to the page (not shown). Thepresence of the sidewall also helps prevent short-circuits betweensource/drain contacts (not shown) and the gate contact 1402.

It is to be understood that aspects of the present invention will bedescribed in terms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps can be varied within the scope of aspects of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”,as well as other variations thereof, means that a particular feature,structure, characteristic, and so forth described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrase “in one embodiment” or “in an embodiment”, as well anyother variations, appearing in various places throughout thespecification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Referring now to FIG. 15, a method for forming a semiconductor device isshown. Block 1502 etches a gap 302 in the upper dielectric layers 202and 204 above the sidewall spacer of dummy gate 110. Block 1504 thenanisotropically etches away the dielectric material (e.g., using RIE)below the gap 302 to expose the side of the dummy gate 110. Block 1506etches the dummy gate 110 laterally using an isotropic etch (e.g., a wetor dry chemical etch) such that spacer layer 112 overhangs the dummygate 110.

Block 1508 nitridizes the sidewall of the dummy gate 110, formingnitridized sidewall 702. Block 1510 then etches away the dummy gate 110,leaving the nitridized sidewall intact. A dummy gate dielectric 108 isreplaced by a final gate dielectric layer 1002 formed from, e.g., ahigh-k dielectric material in block 1512, and a gate 1102 is formed overthe gate dielectric layer 1002 in block 1514. Block 1516 formspassivating dielectric layer 1302 over the gate, with block 1518 formingcontacts through the passivating dielectric layer 1302. The contacts areprevented from short-circuiting to other components on the device by thesidewall spacer 702.

Having described preferred embodiments of a self-aligned spacer forcut-last transistor fabrication (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method of forming a semiconductor device,comprising: laterally etching a dummy gate to recess the dummy gateunderneath a spacer layer, such that the spacer layer overhangs thedummy gate; nitridizing a sidewall of the dummy gate; etching away thedummy gate without removing the nitridized sidewall; forming a gatewithin a boundary defined by the nitridized sidewall; and forming aconductive contact to the gate.
 2. The method of claim 1, wherein thenitridized sidewall has a uniform thickness along its height.
 3. Themethod of claim 1, wherein the dummy gate comprises polysilicon andwherein nitridizing the sidewall converts a depth of the dummy gate tosilicon nitride.
 4. The method of claim 1, further comprising forming apassivating dielectric layer over the gate before forming the conductivecontact.
 5. The method of claim 4, further comprising forming a powerrail in contact with the nitridized sidewall.
 6. The method of claim 5,wherein forming the power rail comprises etching a hole in thepassivating dielectric.
 7. The method of claim 1, further comprisingetching away a sidewall spacer from around a dummy gate before laterallyetching the dummy gate.
 8. The method of claim 1, further comprisingetching away a dummy gate dielectric after etching away the dummy gate.9. The method of claim 8, further comprising forming a gate dielectricover one or more semiconductor fins and over the nitridized sidewallafter etching away the dummy gate.
 10. The method of claim 9, whereinetching away the dummy gate dielectric leaves a dummy gate dielectricremnant directly underneath the nitridized sidewall.
 11. The method ofclaim 10, wherein the dummy gate dielectric is formed from a differentmaterial than the gate dielectric.
 12. A method of forming asemiconductor device, comprising: etching away a sidewall spacer formedon a polysilicon dummy gate; laterally etching the dummy gate to recessthe dummy gate underneath an upper spacer layer, such that the upperspacer layer overhangs the dummy gate; nitridizing a sidewall of thedummy gate to form a silicon nitride sidewall having a uniformthickness; etching away the dummy gate without removing the nitridizedsidewall; etching away a dummy gate dielectric, leaving a dummy gatedielectric remnant directly underneath the nitridized sidewall; forminga gate dielectric over one or more semiconductor fins and over thenitridized sidewall; forming a gate within a boundary defined by thenitridized sidewall; forming a passivating dielectric layer over thegate; forming a conductive contact to the gate; and forming a power railin contact with the nitridized sidewall.
 13. The method of claim 12,wherein the dummy gate dielectric is formed from a different materialthan the gate dielectric.
 14. The method of claim 12, wherein formingthe conductive contact and the power rail comprise etching respectiveholes in the passivating dielectric layer.
 15. A method of forming asemiconductor device, comprising: etching away a sidewall spacer fromaround a dummy gate; laterally etching a dummy gate after etching awaythe sidewall spacer to recess the dummy gate underneath a spacer layer,such that the spacer layer overhangs the dummy gate; nitridizing asidewall of the dummy gate to form a silicon nitride sidewall having auniform thickness; etching away the dummy gate without removing thenitridized sidewall; forming a gate within a boundary defined by thenitridized sidewall; and forming a conductive contact to the gate. 16.The method of claim 15, wherein the dummy gate comprises polysilicon andwherein nitridizing the sidewall converts a depth of the dummy gate tosilicon nitride.
 17. The method of claim 15, further comprising forminga passivating dielectric layer over the gate before forming theconductive contact.
 18. The method of claim 17, further comprisingforming a power rail in contact with the nitridized sidewall.
 19. Themethod of claim 18, wherein forming the power rail comprises etching ahole in the passivating dielectric.
 20. The method of claim 15, furthercomprising etching away a dummy gate dielectric after etching away thedummy gate, leaving a dummy gate dielectric remnant directly underneaththe nitridized sidewall.